1. Field of the Invention
This invention relates to optical data telecommunications. Specifically, this invention relates to an apparatus and method for transmitting data in optical data telecommunications systems and generating data-modulated optical pulse trains for use in such systems. The invention also finds applicability in the field of optical computing.
2. The Related Art
Pulse shaping methods allowing synthesis of complex femtosecond optical waveforms according to specification are well known in the art. As usually practiced, the output waveform is determined by the Fourier transform (xe2x80x9cFTxe2x80x9d) of a spatial pattern transferred by a mask or a modulator array onto the dispersed optical spectrum. As an example, the FT optical pulse shaper is widely used for synthesis of complex femtosecond waveforms. In this geometry, the temporal profile of the output waveform is given by the FT of the mask pattern that is transferred onto the optical frequency spectrum of the pulse. FT pulse shaping was first demonstrated for use in simple pulses of tens of picoseconds in duration. Pulse shaping was then extended to the sub-100 femtosecond (fs) (10xe2x88x9215 second) time scale and demonstrated highly structured waveforms using microlithographically patterned pulse shaping masks. The introduction of liquid crystal modulator arrays and acousto-optic (A/O) modulators into FT pulse shapers led to computer programmable pulse shaping, with millisecond and microsecond reprogramming times, respectively, and widespread adoption of this technique.
FT pulse shapers work well in areas of short-pulse optics. However, for applications in the field of high-bit-rate data telecommunications (i.e.,  greater than 109 bits/second), use of the FT pulse shaper for the generation of xe2x80x98pulse packetsxe2x80x99 presents various difficulties. If a user wants to use the FT pulse shaper to generate pulse-packets (i.e. sequences of pulses where each pulse corresponds to a xe2x80x98bitxe2x80x99 in a data sequence) with a fast (sub-nanosecond) frame update rate, the FT of the desired pulse sequence must be calculated and applied to the pulse shaping modulator array at a rate equal to or greater than the frame-update rate. In general, calculating a FT every nanosecond (or faster) is both difficult and undesirable. Further, in general, a complex modular array is required in order to map the required FT onto the optical frequency spectrum of the short pulse.
It would therefore be high desirable to have a direct (rather than an FT) mapping between a spatial pattern and the resultant ultrafast optical waveform for use in high-bit-rate data telecommunications. As processor speed and data telecommunications network bandwidth increases, photonics will offer significant advantages in high performance computing systems. The Input/Output (I/O) subsystem will be the first area where photonic technologies will have a major impact in the high performance computing environment. Interconnect technology, at either the system or processor level, will be enhanced by combining multiple fast electronic signals onto a single (or multiple) ultrafast (exceeding 1010 bits/second) optical channel for intermodule communication. In high performance computing systems, combining output data words from fast electrical interfaces and serializing them for transfer over an ultrafast optical channel to other high performance systems is a key application where photonics will play a significant role. For example, one may want to convert a parallel electronic data word to an ultrafast optical serial data packet by using a suitable pulse shaping geometry containing an optoelectronic modular array driven by the data word (i.e., in parallel). Thus, it is beneficial and desirable that each bit in the output optical data packet be associated with a single modulator element, both for simplicity and because the need to compute an FT before setting the state of the modulator array would restrict operation to relatively low packet rates.
Research in increasing the data rate per fiber in optical networking has focused on time-division multiplexed (xe2x80x9cTDMxe2x80x9d) and wavelength-division multiplexed (xe2x80x9cWDMxe2x80x9d) transmission. In general, the TDM systems can operate at a data rate of up to about 100 Gbits/s or more per channel. WDM systems operate on at least two channels (typically four or more) in parallel although each operates typically at a lower rate than TDM systems, generally 2.5 up to about 10 Gbits/s. Until recently, there has been little progress in combining TDM and WDM topologies to form a hybrid high channel rate, large number of channels per fiber system. Research has focused on either increasing the speed of a single channel per fiber, or accepting a xe2x80x98lowxe2x80x99 rate while working to radically increase the number of channels per fiber.
Thus, there is an ongoing need for further improvements in optical data telecommunications to increase the data rate per fiber in optical networking systems to beyond 100 Gbits/s.
These and other features and advantages of the present invention will be presented in more detail in the following specification of the invention and in the associated figures.
Direct space-to-time pulse shaping and optical pulse train generation is achieved in the present invention by first generating a controlled pulsed light beam and applying the beam to a pixelator such as a mask or a diffractive optical element (DOE) to obtain a pixelated beam. The spatially patterned beams generated by the pixelator is imaged onto a planar modulator. The individual spot size and spacing from the pixelation optics as well as the imaging system magnification are selected to match the linear array of spots from the pixelation optics to a one-dimensional amplitude modulator arrayxe2x80x94one spot per modular array element. The final operation of the mask generation optics is to image the one-dimensional spatial pattern reflected from (or transmitted thereof) the modulator array onto a spectral dispersing element such as a diffraction grating. The mask generation optics generates a one-dimensional array of spots on the spectral dispersing element where the relative xe2x80x9cheightxe2x80x9d or average power of each spot can be controlled by an individual element in a high-speed optoelectronic modulator array. In this way, different spatial patterns (xe2x80x9cpulse packetsxe2x80x9dxe2x80x94a sequence of optical spots in a one-dimensional array) can be manipulated on the surface of the spectral dispersing element by electronically controlling the optical reflectivity/transmissivity state of individual elements of the optoelectronic modulator array. The pulse shaping is accomplished by applying the light diffracted by the spectral dispersing element to a lens and then to a thin slit. The beam passing through the thin slit may then be applied to an appropriate photonic transmission medium (air-vacuum, fiber optic cable, etc.) for transmission to a receiver where it may be demodulated by a suitably fast photo diode, other conventional photonic receiver, or various optical demodulation devices.